This invention relates to communication signaling systems and, more particularly, to integrated circuit timing circuits which reduce the size and cost of the commonly employed circuitry for checking signal validity in multifrequency signal receivers.
In the field of telephony, signal receivers employing tone detectors are often of the multifrequency type requiring apparatus capable of responding only to coincidental signaling bursts of certain disparate frequency pairs. U.S. Pat. No. 3,281,790, issued to L. C. J. Roscoe on Oct. 25, 1966, is illustrative of such a receiver. In prior art receivers, such as that disclosed by Roscoe, for example, various circuit combinations are employed to test the validity of incoming combinations of coincident two-tone bursts in order to ensure that the output signals of the receiver are generated only in response to valid input signals. The timing circuitry then delivers two different outputs. One output is of fixed duration, independent of the incoming tone burst duration. The duration of the second output is conditionally extended past the termination of the input signal and is unaffected by interruptions in the input signal which do not exceed a predetermined interval.
With the advent of electronic central office and Private Branch Exchange (PBX) systems employing solidstate and electronic switching devices, the emphasis in the design of multifrequency signal receivers is toward compatible integrated circuits with low voltage and current requirements. Heretofore, the timing circuits used to check the validity of incoming signals and to generate responsive outputs have utilized a combination of analog circuitry comprising discrete active devices and passive components. Such discrete timing circuits tend to be bulky, costly and require rather large powering potentials.
It is, accordingly, a primary object of the present invention to provide digital integrated circuitry for performing the various timing functions in multifrequency receivers.
It is a related object of the invention to provide a digital timing circuit which utilizes standard integrated circuit components that operate reliably on low-powering potentials.
A further object of the invention is to reduce the size and cost of timing circuits employed in multifrequency receivers by efficient use of integrated circuit technology.